Clonal cell lines have been established that contain defined numbers of newly integrated MMTV genomes and have identified phenotypes with respect to the induction of viral RNA expression by glucocorticoid hormones. Analysis of viral RNA metabolism in these cells and in other lines transfected with cloned MMTV DNA is underway. Cloned viral DNA has also been evaluated as a template in an in vitro transcription system, and specific initiation of viral RNA synthesis has been demonstrated. Correlation of viral RNA transcription, both in vivo and in vitro, with potential regulatory information in the DNA nucleotide sequence is the established goal of the program.